Tracked agricultural combines are a common type of working vehicle. In these prior combines, left and right crawlers or track assemblies are respectively driven by left and right hydrostatic transmission device (hereinafter abbreviated to HST) and steering is conducted by using independent left and right operating levers linked to a trunnion lever of an associated HST. This form of control causes these combines to be difficult to operate smoothly. They are poor in maneuverability because speed changing operations and turning operations have to be made by using the two levers.
Under the circumstances disclosed in Japanese patent publication No. sho 40-18576, speed changing means such as acceleration pedal and turning means such as steering wheel are respectively linked to each trunnion lever of each HST on the left and the right and forward/backward movement is made by using speed changing means and left/right turning movement is made by using turning means in order to conduct operation smoothly.
However, in the conventional art, many kinds of cams are needed to link the speed changing means and the turning means to each gear lever on both sides and accordingly many kinds of bevel gears are needed to couple various means with the cams.